Abstract
AbstractThe present case study concerns narrowband electromagnetic emission detected in the distant cusp region simultaneously with upgoing plasma flows. The wave properties match the usual properties of the Pc 1–2 mantle waves: small angle between the wave vector and the magnetic field line, left‐hand polarization, and propagation toward the ionosphere. We report here the first direct wave vector measurement of these waves (about 1.2 × 10− 2 rad/km) through multi spacecraft analysis using the three magnetic components and, at the same time, through single spacecraft analysis based on the refractive index analysis using the three magnetic components and two electric components. The refractive index analysis offers a simple way to estimate wave numbers in this frequency range. Numerical calculations are performed under the observed plasma conditions. The obtained results show that the ion distribution functions are unstable to ion cyclotron instability at the observed wave vector value, due to the large ion temperature anisotropy. We thus show that these electromagnetic ion cyclotron (EMIC) waves are amplified in the distant cusp region. The Poynting flux of the waves is counterstreaming with respect to the plasma flow. This sense of propagation is consistent with the time necessary to amplify the emissions to the observed level. We point out the role of the wave damping at the He++ gyrofrequency to explain that such waves cannot be observed from the ground at the cusp foot print location.
Highlights
The in situ detection of magnetosheath ions in the terrestrial polar cusps has revealed a direct access of shocked solar wind ions to the ionosphere [Heikkila and Winningham, 1971; Frank, 1971; Russell et al, 1971]
We report here the first direct wave vector measurement of these waves through multi spacecraft analysis using the three magnetic components and, at the same time, through single spacecraft analysis based on the refractive index analysis using the three magnetic components and two electric components
We show that these electromagnetic ion cyclotron (EMIC) waves are amplified in the distant cusp region
Summary
The in situ detection of magnetosheath ions in the terrestrial polar cusps has revealed a direct access of shocked solar wind ions to the ionosphere [Heikkila and Winningham, 1971; Frank, 1971; Russell et al, 1971]. HEOS-2 data revealed simultaneity of downward ion flows and ultra-low-frequency (ULF) magnetic noise in the high altitude cusp region [D’Angelo et al, 1974]. These observations have been confirmed by Polar observations [Le et al, 2001] and Cluster observations [Nykiri et al, 2004, 2006; Grison et al, 2005]. The observed waves, so-called mantle waves, are narrowband with a clear left-hand polarization and a small propagation angle (~20°) These waves can be observed in the distant mantle simultaneously with unstable distribution functions to the ion cyclotron instability [Engebretson et al, 2012]. We discuss our results and the possibility for the distant cusp to be the source region of the Pc 1–2 waves observed in the plasma mantle and on the ground
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